Optical spectroscopy systems commonly used to analyze the spectrum of a light radiation generally utilize prisms or gratings which give rise to a spatial dispersion of the various wavelengths present in the radiation to be analyzed. In certain applications very high resolutions are required in order to separate wavelengths which differ e.g. by some nanometers as may be necessary to characterize a monochromatic or quasi-monochromatic source (namely an LED or a laser diode), or in Raman or Brillouin spectroscopy.
Obtaining such high resolutions by spatially dispersive means with satisfactory results demands use of very cumbersome, complicated and expensive systems. To overcome this problem, spectroscopy systems have been proposed which exploit different means for the selection of the frequency range of interest, such as for instance interference filters.
Interference filters, as is known, consist of a transparent dielectric substrate, with a suitable refractive index, onto which a complex multilayer coating has been deposited. Light radiation traversing the filter undergoes multiple reflections at the interfaces between the various layers. By an appropriate choice of the refractive indices and thicknesses of the layers, a certain portion of the incident radiation spectrum can be transmitted or eliminated by interference. The cut-off wavelength (in case of high-pass or low-pass filters), or the central wavelength of the transmitted or eliminated band (in case of bandpass or band-elimination filters) varies with the incidence angle, since the optical paths of the various rays inside the filter change.
An example of system using an interference filter is described in WO-A-90/07108 published on Jun. 28, 1990.
That document discloses a Raman spectroscopy apparatus where a sample is illuminated by light from a laser source, which is reflected to it by a dichroic mirror, and a bidimensional image of the illuminated area is formed on a detector through a suitable optical system. On the way to the detector, the light passes through an interference filter which selects a desired line from a Raman spectrum scattered by the sample. The filter is arranged for pivotal movement about an axis perpendicular to the optical axis, to scan in wavelength the scattered spectrum.
For each position of the filter, the rays or beams which give rise to the image traverse the interference filter at different angles. Hence the image is a non-monochromatic image of the sample, and each point on the detector will be associated with a point of the sample and a wavelength. A computer measures the frequencies and the relative intensities of the peaks present in the signals supplied by the various detector points and associates the results with the spectra of the various molecules. The same computer can control the filter movements.
The known system has a number of drawbacks which limit its performance. More particularly, the interference filter is used basically as a monochromator, and hence its resolution is strictly dependent on the width of the filter passband. To obtain good resolution not only must the band must be very narrow, but the corresponding peak must also be isolated from adjacent secondary peaks, if any. It is rather complicated and hence expensive to fabricate interference filters meeting these requirements. Besides, the resolution also depends on the accuracy with which the amplitude of the filter angular displacements filter can be determined. Since the cost of angular position measuring devices increases with sensitivity, also such requirement causes an increase in the system costs. Finally the presence of moving parts generally gives rise to reliability problems.